The present invention relates to a pneumatic flow control system for maintaining a constant, low flow rate, particularly a system for drawing air through a sampling device at a constant, controllable, low flow rate.
Prior art systems for drawing air through a sampling device are disclosed in U.S. Pat. Nos. 4,904,987, 4,896,548, 4,747,315, 4,576,054, 4,532,814, 4,527,953, 4,432,248, 4,269,059, 4,257,746, 4,123,932 and 4,063,824. In some of these systems, a constant, low flow rate is not maintained. In others, a constant, low flow rate is achieved by controlling the speed of a pump and/or by shunting air around a pump. The prior art systems have proven to be less than fully satisfactory.